1. Field of the Invention
This invention relates to a device and method for separating heavier and lighter fractions of a fluid sample. More particularly, this invention relates to a device and method for collecting and transporting fluid samples whereby the device and fluid sample are subjected to centrifugation in order to cause separation of the heavier fraction from the lighter fraction of the fluid sample.
2. Description of Related Art
Diagnostic tests may require separation of a patient""s whole blood sample into components, such as serum or plasma, the lighter phase component, and red blood cells, the heavier phase component. Samples of whole blood are typically collected by venipuncture through a cannula or needle attached to a syringe or an evacuated collection tube. Separation of the blood into serum or plasma and red blood cells is then accomplished by rotation of the syringe or tube in a centrifuge. Such arrangements use a barrier for moving into an area adjacent the two phases of the sample being separated to maintain the components separated for subsequent examination of the individual components.
A variety of devices have been used in collection devices to divide the area between the heavier and lighter phases of a fluid sample.
The most widely used device includes thixotropic gel materials such as polyester gels in a tube. The present polyester gel serum separation tubes require special manufacturing equipment to prepare the gel and to fill the tubes. Moreover, the shelf-life of the product is limited in that overtime globules may be released from the gel mass. These globules have a specific gravity that is less than the separated serum and may float in the serum and may clog the measuring instruments, such as the instrument probes used during the clinical examination of the sample collected in the tube. Such clogging can lead to considerable downtime for the instrument to remove the clog.
No commercially available gel is completely chemically inert to all analytes. If certain drugs are present in the blood sample when it is taken, there can be an adverse chemical reaction with the gel interface.
Therefore, a need exists for a separator device that (i) is easily used to separate a blood sample; (ii) is independent of temperature during storage and shipping; (iii) is stable to radiation sterilization; (iv) employs the benefits of a thixotropic gel barrier yet avoids the many disadvantages of placing a gel in contact with the separated blood components; (v) minimizes cross contamination of the heavier and lighter phases of the sample during centrifugation; (vi) minimizes adhesion of the lower and higher density materials against the separator device; (vii) is able to move into position to form a barrier in less time than conventional methods and devices; (viii) is able to provide a clearer specimen with less cell contamination than conventional methods and devices; and (ix) can be used with standard sampling equipment.
The present invention is a method and assembly for separating a fluid sample into a higher specific gravity phase and a lower specific gravity phase. Desirably, the assembly of the present invention comprises a plurality of constituents. Preferably, the assembly comprises a container and a composite element.
Most preferably, the container is a tube and the composite element is a separator arranged to move in the tube under the action of centrifugal force in order to separate the portions of a fluid sample.
Most preferably, the tube comprises an open end, a closed end and a sidewall extending between the open end and closed end. The sidewall comprises an outer surface and an inner surface. The tube further comprises a closure disposed to fit in the open end of the tube with a resealable septum. Alternatively, both ends of the tube may be open, and both ends of the tube may be sealed by elastomeric closures. At least one of the closures of the tube may include a resealable septum.
Preferably, the separator element is releaseably positioned at the open end of the tube with the closure. Alternatively, the separator element may also be releasably positioned at the closed end of the tube.
Preferably, the closure may further include a bottom recess that extends into the tube having a plurality of inwardly extending circumferentially spaced flexible walls or a flexible full ring for holding the separator.
Preferably, the separator element comprises an overall specific gravity at a target specific gravity of "sgr"t. The target specific gravity is that required to separate a fluid sample into at least two phases.
Preferably, the separator comprises at least two or more regions of differing specific gravities. Preferably, at least one of the regions is higher than the target specific gravity and at least one of the regions is lower than the target specific gravity.
Preferably, the separator element comprises a toroid or a bellows, a foam or a float and a sinker or a ballast. The bellows comprises opposed first and second ends and a seal body extending between the ends. The outer diameter of the seal body is larger than the inner diameter of the tube for sealing engagement. Most preferably, the seal body has elastic properties.
Most preferably the float is securely mounted to the first end of the bellows and the ballast is securely mounted to the second end of the bellows.
Alternatively, the bellows comprises a first end that is a resealable septum and an open second end.
Preferably, the separator may be initially located at any position within the tube. Most preferably, the separator is held in position at the top of the tube by an interference fit between the seal body and the tube inner diameter.
Preferably, the separator has central passageway that extends from the first end through the seal body and to the second end of the bellows.
Preferably, the bellows has a specific gravity of about 0.8 to about 1.2. Most preferably, the bellows is made from an elastomer which has a 50% tensile modulus (YOUNGS) from about 100 psi to about 500 psi.
Desirably, the seal body may be comprised of any natural or synthetic elastomer or mixture thereof, that are inert to the fluid sample of interest and is flexible.
Preferably, the seal body comprises a qualitative stiffness, expressed as follows:       S    *    =      k          a      ⁢              xe2x80x83            ⁢              ρ        w            ⁢              D        2            
whereby S* is the non-dimensional stiffness coefficient, k is a force required to deflect the bellows a given length, a is the applied acceleration, D is the diameter of the seal body and xcfx81w is the density of water.
Desirably, the qualitative stiffness of the seal body is from about 0.00006 to about 190.
Preferably, the seal body may be subjected to a characteristic or radial deflection under an applied load, such as an axially applied load. The characteristic or radial deflection is defined as a change in length of the seal body relative to the change in cross section diameter of the seal body. Preferably, the seal body has a characteristic or radial deflection ratio of about 1.5 to about 3.5.
Preferably, the seal body when subjected to an applied load, such as centrifugation, to cause axial deformation of the seal body, the change in cross section diameter may be expressed as follows:                               D          before                -                  D          during                            D        before              xc3x97    100    ⁢    %    =      Δ    ⁢          xe2x80x83        ⁢          D      m      
wherein xcex94Dm is from about 5% to about 20%.
Therefore, a change in cross section diameter of the seal body is proportional to the undeflected cross section diameter of the seal body. Preferably, the proportion is from about 0.03 to about 0.20.
Desirably, the ballast is a substantially rigid moldable thermoplastic material such as polyvinyl chloride, polystyrene, polyethylene, polypropylene, polyethyleneterethalate, stainless steel, polyester and mixtures thereof that are inert to the fluid sample of interest. Most preferably, the ballast is a high density material. Preferably, the ballast is mounted around the second end of the bellows so as not to interfere with the central passageway of the separator. Most preferably, the ballast has a useful specific gravity from about 1.1 to about 7.9.
Desirably, the float is attached to the first end of the bellows whereby the float is in direct communication with the central passageway. Preferably, the float comprises small holes to bleed the air out of the central passageway of the separator. Most preferably, the float has a density from about 0.06 to about 0.95. Preferably, the float is a low density material such as polyethylene, polypropylene, polystyrene, foam, an air encapsulated system or a mixture of materials that reseal.
Preferably, the separator has an aggregate specific gravity of about 1.028 to about 1.09 g/cc so that the separator will come to rest under centrifugal force substantially at the border between the heavier and lighter phases of a fluid sample under consideration.
Preferably, the separator as a whole will function under load created by an applied acceleration from about 300 g to about 3000 g.
Preferably, the separator is initially secured to the top area of the tube and in alignment with the closure. The separator is fitted at the top end of the tube whereby the bellows of the separator, which provides the largest diameter of the separator in its undeformed state, may have an interference fit with the inner surface of the sidewall of the tube.
In use, a fluid sample enters the assembly by needle. The needle penetrates the closure and the float of the separator. The sample enters the assembly through the needle and through the central passageway of the bellows and then into the body of the tube. The needle is withdrawn from the assembly and the septum of the closure and the float reseals.
The assembly is then subjected to centrifugation. Forces exerted by the centrifuge cause the seal body to separate from the inner wall of the tube whereby the seal body elongates due to the difference in the buoyancy of the different elements of the separator. Under centrifugation, the separator migrates axially down the tube towards the closed end to the desired interface.
Sufficient movement of the separator will cause the separator to contact the blood. The ballast at the second end of the bellows moves axially downward under the centrifugal loading. The optional air bleed holes in the float or the resealable septum of the bellows serve to control the descent rate of the separator into the fluid sample.
Following immersion of the separator in the fluid, the float provides a buoyant upward force on the separator due to the displaced fluid. Simultaneously, the ballast provides an axial force downward on the separator. The combined forces stretch the seal body axially causing radial movement of the seal body inwardly which pulls it out of contact with the inner wall of the tube so that it is free to move axially without any frictional drag.
Therefore, a path is developed between the inner wall of the tube and the separator that permits the flow of the low-density component past the separator as it migrates down the tube. Migration of the separator terminates when it reaches the position between the lower density fluid component and higher density fluid or cellular/solid components, equal to its overall density. Upon terminating centrifugation, the seal body expands to its undeformed shape, sealing against the inner wall of the tube, thereby creating a barrier between the higher and lower density components of the sample fluid.
The separator""s position at the top of the tube in alignment with the closure and the separator""s float and central passageway, provides easy direct loading of the fluid sample into the tube. Thus, the fluid sample is easily delivered into the tube without exposing the uncentrifuged fluid sample to the outer surface area of the separator.
When the fluid sample is blood, the higher specific gravity portion that contains the cellular components is between the separator and the bottom of the tube after centrifugation. The lower specific gravity portion that contains the cell free serum or plasma fraction is between the float of the separator and the top of the tube.
The separator of the present invention comprises a useful range of parameters and there are two principle driving equations for defining the parameters:
"sgr"tVt="sgr"fVf+"sgr"sVs
(Conservation of Mass)             (                                    (                                          σ                f                            -                              σ                t                                      )                    ⁢                      V            f                          -                              (                                          σ                s                            -                              σ                t                                      )                    ⁢                      V            s                              )        ⁢          ρ      w        =                    δ        ·        Δ            ⁢              xe2x80x83            ⁢              D        ·        k              a  
(Force Balance)
The following non-dimensional parameters may then be substitute into the force balance:
Vs*=Vs/D3; Vf*=Vf/D3; S*=k/a xcfx81wD2
to arrive at:       (                            (                                    σ              f                        -                          σ              t                                )                ⁢                  V          f          *                    -                        (                                    σ              s                        -                          σ              t                                )                ⁢                  V          s          *                      )    =                    δ        ·        Δ            ⁢              xe2x80x83            ⁢              D        ·                  S          *                      D  
So as to scale prototypes to any size device, wherein the following are defined:
"sgr"t, "sgr"f, "sgr"s are the specific gravities of the bellows, float and ballast, respectively;
Vt, Vf, Vs are the volumes of the bellows, float and ballast, respectively;
xcfx81w is the density of water;
k is the separator spring constant;
a is the applied acceleration; and
xcex4 is the deflection ration defined by: xcex94L/xcex94D, where xcex94L is the change in length.
The left side of the equation can be an infinite number of combinations of materials and geometries and if it is equal to the product of the right side it can be concluded that the device will function.
Desirable values for the right side of the equation are as follows:
xcex4=1.5-3.5
xcex94D/D=0.05 to 0.2
S*=0.043 to 0.220.
Alternatively, the separator element may comprise an arrangement comprising a bellow member, a ballast member and a buoyancy or a float member.
Most preferably, the bellow member is made of a material and shape which allows deflections caused by opposing forces.
Most preferably, the buoyancy member has a component density whereby it has the capability of floating in serum of a blood sample. Preferably, the buoyancy member is made of a low density material such as foam or a material or mixture of materials so that it simulates a low density material such as foam.
Most preferably, the ballast member has a component density whereby it has the capability of sinking in a blood sample. Preferably, the ballast member is made of a high density material such as a substantially rigid moldable thermoplastic material. Such materials include but are not limited to polyvinyl chloride, polystyrene, polyethylene, polypropylene, stainless steel, polyester and mixtures thereof that are inert to the fluid sample of interest.
Most preferably, the separator element is arranged whereby the ballast member and buoyance members are connected and a central passageway extends through them. The bellow member covers the entrance to the central passageway and provides a pierceable barrier extending across the entrance to the central passageway.
Most preferably, the separator elements are assembled to create opposing forces to deflect the bellow member inwardly and allow it to move axially in the tube while under the proper loading.
Most preferably, the overall density of the separator is the target density "sgr"t whereby to cause the device to position itself between the higher and lower density of a fluid sample.
Desirably, the bellow member may be comprised of any natural or synthetic elastomer or mixture thereof, that are inert to the fluid sample of interest and is flexible.
Preferably, the bellow member comprises a qualitative stiffness, expressed as follows:       S    *    =      k          a      ⁢              xe2x80x83            ⁢              ρ        w            ⁢              D        2            
whereby S* is the non-dimensional stiffness coefficient, k is a force required to deflect the bellow member a given length, a is the applied acceleration, D is the diameter of the bellow member and xcfx81w is the density of water.
Desirably, the qualitative stiffness of the bellow member is from about 0.00006 to about 190.
Preferably, the bellow member may be subjected to a characteristic or radial deflection under an applied load, such as an axially applied load. The characteristic or radial deflection is defined as a change in length of the bellow member relative to the change in cross section diameter of the bellow member. Preferably, the bellow member has a characteristic or radial deflection ratio of about 1.5 to about 3.5.
Preferably, the bellow member when subjected to an applied load, such as centrifugation, to cause axial deformation, the change in cross section diameter may be expressed as follows:                               D          before                -                  D          during                            D        before              xc3x97    100    ⁢    %    =      Δ    ⁢          xe2x80x83        ⁢          D      m      
wherein xcex94Dm is from about 5% to about 20%.
Therefore, a change in cross section diameter of the bellow member is proportional to the undeflected cross section diameter of the bellow member. Preferably, the proportion is from about 0.03 to about 0.20.
Desirably, the ballast member is a substantially rigid moldable thermoplastic material such as polyvinyl chloride, polystyrene, polyethylene, polypropylene, polyester and mixtures thereof that are inert to the fluid sample of interest. Most preferably, the ballast member is a high density material. Most preferably, the ballast member has a useful specific gravity from about 1.1 to about 7.9.
Desirably, the buoyancy member has a useful specific gravity from about 0.06 to about 0.95. Preferably, the buoyancy member is a low density material such as foam or encapsulated air.
Preferably, the separator has an aggregate specific gravity of about 1.028 to about 1.09 g/cc so that the separator will come to rest under centrifugal force substantially at the border between the heavier and lighter phases of a fluid sample under consideration.
Preferably, the separator as a whole will function under load created by an applied acceleration from about 300 g to about 3000 g.
Preferably, the separator is initially secured to the bottom recess of the closure. The separator is fitted with the closure whereby the bellows member of separator, which provides the largest diameter of the separator in its undeformed state, has a fit with the bottom recess of the closure. Alternatively, the separator may also be releasably positioned at the closed end of the tube.
In use, a fluid sample enters the assembly by needle. The needle penetrates the closure and the bellow member of the separator. The sample enters the assembly through the needle and through the central passageway of the separator and then into the body of the tube. The needle is withdrawn from the assembly and the septum of the closure and the bellow member reseals.
The assembly is then subjected to centrifugation. Forces exerted on the separator by the centrifuge cause the separator to separate from the closure or move from its initial position whereby the bellow member elongates as the separator migrates due to the forces pulling on it. Under centrifugation, the separator is released from the closure. The separator migrates axially down the tube towards the closed end.
Sufficient movement of the separator will cause the separator to contact the blood. Air trapped in the central passageway creates a buoyancy that could prevent further sinking of the separator into the fluid. However, the trapped air vents through a defect in the bellow member that is caused by the needle. This venting of air permits further movement of the separator into the fluid.
Following immersion of the separator in the fluid, the buoyancy member provides a buoyant upward force on the separator due to the displaced fluid. Simultaneously, the ballast member provides an axial force downward on the separator. The combined forces stretch the bellow member axially and pulls it out of contact with the closure so that it is free to move axially without any frictional drag.
Therefore, a path is developed between the inner wall of the tube and the separator that permits the flow of the low-density component past the separator as it migrates down the tube. Migration of the separator terminates when it reaches the position between the lower density fluid component and higher density fluid or cellular/solid components, equal to its overall density. Upon terminating centrifugation, the bellow member expands to its undeformed shape, sealing against the inner wall of the tube, thereby creating a barrier between the higher and lower density components of the sample fluid.
The separator""s position at the top of the tube in alignment with the closure and the separator""s penetrable bellows member and central passage, provides easy direct loading of the fluid sample into the tube. Thus, the fluid sample is easily delivered into the tube without exposing the uncentrifuged fluid sample to the outer surface area of the separator.
When the fluid sample is blood, the higher specific gravity portion that contains the cellular components is between the separator and the bottom of the tube after centrifugation. The lower specific gravity portion that contains the cell free serum or plasma fraction is between the bellows of the separator and the top of the tube.
The separator of the present invention comprises a useful range of parameters and there are two principle driving equations for defining the parameters:
"sgr"tVt="sgr"fVf+"sgr"sVs
(Conservation of Mass)             (                                    (                                          σ                f                            -                              σ                t                                      )                    ⁢                      V            f                          -                              (                                          σ                s                            -                              σ                t                                      )                    ⁢                      V            s                              )        ⁢          ρ      w        =                    δ        ·        Δ            ⁢              xe2x80x83            ⁢              D        ·        k              a  
(Force Balance)
The following non-dimensional parameters may then be substitute into the force balance:
Vs*=Vs/D3; Vf*=Vf/D3; S*=k/axcfx81wD2
to arrive at:       (                            (                                    σ              f                        -                          σ              t                                )                ⁢                  V          f          *                    -                        (                                    σ              s                        -                          σ              t                                )                ⁢                  V          s          *                      )    =                    δ        ·        Δ            ⁢              xe2x80x83            ⁢              D        ·                  S          *                      D  
So as to scale prototypes to any size device, wherein the following are defined:
"sgr"t, "sgr"f, "sgr"s are the specific gravities of the bellow member, buoyance member and ballast member, respectively;
Vt, Vf, Vs are the volumes of the bellow member, buoyance member and ballast member, respectively;
xcfx81w is the density of water;
k is the separator spring constant;
a is the applied acceleration; and
xcex4 is the deflection ration defined by: xcex94L/xcex94D, where xcex94L is the change in length.
The left side of the equation can be an infinite number of combinations of materials and geometries and if it is equal to the product of the right side it can be concluded that the device will function.
Desirable values for the right side of the equation are as follows:
xcex4=1.5-3.5
xcex94D/D=0.05 to 0.2
S*=0.043 to 0.220.
The assembly of the present invention is advantageous over existing separation products that use gel. In particular the assembly of the present invention will not interfere with analytes as compared to gels that may interfere with analytes. Another attribute of the present invention is that the assembly of the present invention will not interfere with therapeutic drug monitoring analytes.
Most notably, is that the time to separate a fluid sample into separate densities is achieved in substantially less time with the assembly of the present invention as compared to assemblies that use gel.
Another notable advantage of the present invention is that fluid specimens are not subjected to low density gel residuals that are at times available in products that use gel.
A further attribute of the present invention is that there is no interference with instrument probes.
Another attribute of the present invention is that samples for blood banking tests are more acceptable than when a gel separator is used.
Another attribute of the present invention is that only the substantially cell-free serum fraction of a blood sample is exposed to the top surface of the separator, thus providing practitioners with a clean sample.
Additionally, the assembly of the present invention does not require any additional steps or treatment by a medical practitioner, whereby a blood or fluid sample is drawn in the standard fashion, using standard sampling equipment.